network: make check-pointer-deref clean

Add the needed assert changes to make the code clean
for the new check-pointer-deref script.

src/network/networkd-dhcp-server.c

@@ -326,6 +326,7 @@ void manager_toggle_dhcp4_server_state(Manager *manager, bool start) {
 
 static int dhcp_server_find_uplink(Link *link, Link **ret) {
         assert(link);
+        assert(ret);
 
         if (link->network->dhcp_server_uplink_name)
                 return link_get_by_name(link->manager, link->network->dhcp_server_uplink_name, ret);